The invention relates to an arc extinguishing chamber of a low-voltage, high-current circuit breaker.
The document EP 0,306,382 describes a multipole circuit breaker with a molded insulating case housing an operating mechanism coupled to a switching bar so as to perform opening and closing of all the poles of the circuit breaker. Each pole comprises a stationary contact means, a movable contact means and an arc extinguishing chamber. The stationary contact means comprise a fixed conducting current input strip supported by the back-plate of the case, stationary main contacts and a stationary arcing contact. The stationary contact strip is arranged in front-connection. In other words, it is formed by a relatively straight metal bar which does not impose a curved path on the current flowing through it and consequently prevents any electromagnetic loop effect on the arc arising when opening takes place. The movable contact means comprise a fixed conducting current input strip also supported by the back-plate of the case, and a contact system having a plurality of identical main contacts arranged in two series of the same number on each side of a movable arcing contact extending longitudinally along the center axis of the pole. The arcing contact protrudes out towards the inside of the chamber and enables the arc to enter the chamber. The arc extinguishing chamber is arranged above the first strip and comprises side flanges supporting a stack of separators formed by metal arc deionization plates, each plate having a V-shaped notch. A lower arcing horn and an upper arcing horn are located on each side of the stack of plates of the extinguishing chamber. The upper arcing horn is provided with two side flaps which are folded back in the direction of the stationary contacts and which partially blank off the upper part of the entrance to the chamber. A lateral arc guide cheek is located in the contact separation zone in the extension of each of the two side flanges of the chamber. The two cheeks protrude out from the plane of the corresponding flange obliquely towards one another. The cheeks have an appreciably trapezoid shape and are situated in the bottom part of the chamber, near to the stationary contact. They limit access to the chamber in the bottom part, near to the lower arcing horn. When opening of the contacts takes place, the guide cheeks enable the arc root to be centered along the lower arcing horn.
Such a chamber is particularly well-suited for circuit breakers of high current rating, about 3000 A, having to comply with the requirements of single-phase breaking tests at relatively high voltage, about 600 V, with a fairly low current intensity of about 5 to 10 times the rated current of the circuit breaker. It is on the other hand quite unsuitable for breaking very high fault currents, of about 100 kA, in a moderately high voltage of 480 V. When the breaking current intensity is very high, the diameter of the cross-section of the arc is in fact large, so that the arc forms almost instantaneously on all the contact fingers and immediately occupies the whole of the available volume of the chamber. The chamber is then subjected to a very high pressure and to a very high temperature. The gas-generating emission of the cheeks contributes even further to increasing the pressure. However the positioning of the cheeks and of the upper arcing horn contributes to reducing the opening of the entrance to the chamber, resulting in a delay in balancing of the pressures in the case of the apparatus, which may cause the latter to explode.
An arc extinguishing chamber for a circuit breaker is furthermore known, described in U.S. Pat. No. 4,650,938, comprising separators designed to be arranged near to the contacts, and a pair of side flanges arranged on each side of a longitudinal mid-plane of the chamber to support the separators. Each flange comprises an elongate rib protruding out towards the opposite flange. The two ribs extend over the whole height of the chamber, perpendicularly to the separators, and facing one another. They are arranged between the separators and the contacts so as to restrict the width of the opening of the entrance of the chamber uniformly over the whole height of the chamber. When opening of the contacts takes place, the ribs modify the gas flow in the chamber and protect the part of the flanges supporting the separators from the flow of hot gases. Such an arrangement is useful for a low-performance, bottom-of-the-range circuit breaker in order to prevent the flanges being damaged by the heat of the arc gases. It does not on the other hand enable the whole of the volume of the chamber to be used to cool the gases. The dimensions of the chamber necessary to dissipate the energy of an arc of given power will therefore be very large. In addition, a circuit breaker of this type requires measures to be taken to facilitate entry of the arc into the chamber, and in particular requires the stationary contact to be U-shaped under the bottom surface of the chamber in order to create an electromagnetic loop effect fostering entry of the arc into the chamber.
The object of the invention is therefore to improve the performances of a multipole low-voltage circuit breaker of high rating equipped with an upper arcing horn. Its object is in particular to enable breaking of electric arcs of large diameter generated by very high fault currents in medium voltages, preventing risks of explosion of the chamber.
According to the invention, this objective is achieved by means of a pole for an electrical circuit breaker comprising a case, and an operating mechanism able to switch from a closed position to an open position, said pole comprising:
a stationary contact means comprising a contact zone,
a movable contact means able to be coupled to said mechanism and to switch from a closed position in which it is in contact with the contact zone of the stationary contact means to an open position where the two contact means are separated,
an arc extinguishing chamber comprising:
two parallel side flanges made of insulating material situated at equal distance from a longitudinal mid-plane of the chamber,
separators extending from one of the side flanges to the other, appreciably perpendicularly to the longitudinal mid-plane,
a front opening situated near to the contact zone of the stationary contact means,
a lower arcing horn made of conducting material, electrically connected to the stationary contact means,
an upper arcing horn made of conducting material, the separators being situated between the lower arcing horn and the upper arcing horn,
a pair of lateral dielectric shields made of electrically insulating material protruding out towards the mid-plane and laterally limiting the front opening of the chamber,
wherein
each lateral dielectric shield is arranged in such a way as to be interposed laterally between the separators and the movable contact means in the open position,
the pair of lateral dielectric shields is arranged in such a way that the width of the opening of the chamber measured perpendicularly to the longitudinal mid-plane is appreciably smaller near to the upper arcing horn than near to the lower arcing horn.
The arcing horns contribute to making the arc enter the chamber as soon as the head of the main electric arc switches onto the upper arcing horn, even if the contact strips are of the front-connection type, without any electromagnetic loop effect on the arc. The dielectric shields for their part enable the electric arc to be elongated and curved between the movable contact in the open position and the separators so as to foster switching of the arc onto the upper arcing horn. The narrowing of the dielectric shields in the bottom part of the chamber, near to the stationary contact, enables a wide opening to be obtained for entry to the chamber, which fosters balancing of the pressures between the chamber and the front volume of the pole. A rapid increase of the pressure in the chamber would in fact be adverse to the arc entering the chamber and remaining therein. It would moreover be liable to cause the chamber to explode. The combination of the profiled dielectric shields and the arcing horns favors elongation of the arc in the chamber while controlling the pressure therein.
Preferably, the upper arcing horn comprises a free end situated near to the movable contact means in the open position and interposed between the separators and the movable contact means in the open position. The upper part of each lateral dielectric shield is interposed between the separators and the free end of the upper arcing horn. This positioning of the upper arcing horn enables optimum switching of the arc onto the upper arcing horn to be achieved. The dielectric shields form an obstacle for the arc. The arc has to go round the shields to reach the separators, which elongates the arc.
According to one embodiment, the lateral dielectric shields are made of a material generating little or no gas, in particular a polytetrafluorethylene or a strongly charged 6-6 or 4-6 polyamide. The screens therefore do not contribute to increasing the pressures in the chamber. In addition, they do not generate any gas flow liable to hinder entry of the arc into the chamber.
According to one embodiment, each dielectric shield comprises an upper part and a lower part appreciably narrower than the upper part. Alternatively, the lower part can be totally eliminated.
According to one embodiment, each dielectric shield comprises a front part, the front parts of the two dielectric shields laterally bounding the contact zone at least partially. The front parts of the shields acts as a protective screen for the side wall of the pole.